Space charge limited vacuum electron devices (VEDs) such as triodes, tetrodes, travelling wave tubes and the like (but not including gyrotrons and similar devices) include a directly or indirectly heated cathode which is heated to a high temperature by a filament and includes a low work function electron emitting material such as barium or thoriated tungsten. The electrons are emitted and formed into an electron beam which is generally modulated and amplified by the VED. Such VEDs are used in broadcast transmitters, vacuum tubes, cathode ray tubes, television tubes, satellite ground stations, other forms of communications equipment and other forms of equipment requiring a high-power source of radio frequency (RF) energy.
As the filament voltage is increased in such a VED the temperature of the cathode increases and the emission of electrons generally increases unless the electron emitting material has been exhausted at which point the VED has reached its actual end of life. An increase of electron emission corresponds with a beam current increase as the electrons form the beam current of the VED. As a VED ages, if its filament voltage is held constant it will exhibit a roll off of beam current leading to end of life. Increasing the filament voltage of the VED as it ages will tend to extend the effective service life of the VED to a point.